Liberal Senator Cory Bernardi has hit back at Opposition Leader Bill Shorten, who sought to play down an internal Labor rift by highlighting Senator Bernardi's disputes with his own party.

Mr Shorten has been asked why he has embarked on low-profile campaign events with Labor's number one Senate candidate for the West Australian by-election Joe Bullock.

Mr Bullock, who is from the socially conservative Shop, Distributive and Allied Employees Association (SDA), did not campaign with Mr Shorten when the Opposition Leader was in Perth this week.

Mr Shorten, who is reported to have said elections are less about candidates and more about political parties, has told the Guardian he has done a photo shoot and forum with Mr Bullock.

He also says if the media want to examine "unusual Senate candidates" they should be looking at the South Australian Liberal Senator Cory Bernardi instead of Mr Bullock.

"We are talking about people who are number one on the Senate ticket," he said.

"I think Cory Bernardi as number one on the Senate ticket ... the Liberal Party rushed to disown his views yet they put him up as number one."

Mr Shorten says he has different views to Mr Bullock but finds it "interesting that Cory Bernardi gets a leave pass".

"I am not a monarchist and I voted for gay marriage but I have plenty more disagreements with Cory Bernardi," he said.

"If you want to talk about someone who is number one on a Senate ticket who has repugnant views about things, this [Senator Bernardi] is a guy who says he loves families but he only loves certain kinds of families. He thinks step-families aren't as good as nuclear families.

"If you want to look at unusual Senate candidates, the Libs have got … they're special, the Liberal Party."

Free trade is the oldest argument in federal politics and the issue that literally defined the federation era but opposition exists to the TPP, courtesy of the Investor-State Dispute Resolutions clause.